Infra: Fix Replaces/Superseded-By/Requires links for dirhtml (#2981)

After #2972, when building dirhtml,
we need to re-add the initial / for the links to PEPs
in the Replaces/Superseded-By/Requires headers.

pep_sphinx_extensions/pep_processor/transforms/pep_headers.py

@@ -131,6 +131,8 @@ class PEPHeaders(transforms.Transform):
                 new_body = []
                 for pep_str in re.split(r",?\s+", body.astext()):
                     target = self.document.settings.pep_url.format(int(pep_str))
+                    if self.document.settings.builder == "dirhtml":
+                        target = f"../{target}"
                     new_body += [nodes.reference("", pep_str, refuri=target), nodes.Text(", ")]
                 para[:] = new_body[:-1]  # drop trailing space
             elif name == "topic":